There has been widespread interest in the development of hydrogenated amorphous silicon alloys. Such alloys find application in a variety of devices, although their main application remains in tandem photovoltaic cells (J. M. Ogden, et al., Report at Princeton University Center for Energy and Environmental Study, 231 (February 1989)) and G. H. Lin, et al., Appl. Phys Lett. 55, 386 (1989). Such devices have the capability of producing efficient, economical, large-scale solar to electrical or chemical energy conversion (G. H. Lin, et al., Appl. Phys Lett. 55, 386 (1989)).
However, a solar cell or other device employing an active semiconductor utilizes only a limited portion of the incident energy. For example, photon energy higher than the optical bandgap is not fully utilized and much is wasted as heat, while that lower than the bandgap is not absorbed. Thus, it is of great technological importance to discover semiconductor materials with optical bandgaps that differ from that of amorphous silicon, a substance commonly employed as semiconductor material. Alloying amorphous silicon with various other elements might result in beneficial optical bandgap energies as well as beneficial optical and electrical properties.
In the recent past, several groups have experimented with hydrogenated amorphous silicon alloys for use as semiconductor material. Hydrogenated amorphous silicon alloys have been mainly prepared with the Group IV elements. Thus, alloys such as a-Si:C:H, a-Si:Ge:H, and a-Si:Sn:H have been formed for study (where "a-" indicates amorphous; J. Chevallier, et al., Sol. St. Commun. 24, 867 (1977); K. D. MacKenzie, et al., Phys. Rev. B, 31, 2198 (1985); D. A. Anderson et al., Phil. Mag., 35, 1 (1977); J. Tyczkowski Thin Solid Films, 168, 175 (1989); C. Verie, et al., J. de Phys. Coll. c4, 42, Supp. 10, 667 (1981); and D. Girginoudi, et al., Appl. Phys., 66, 354 (1989)). Except for these materials, very few other amorphous silicon alloys have been characterized, the most widely studied being a-Si:N:H, a-Si:O:H and a-Si:F:H (P. K. Shufflebotham, et al., J. Non. Cryst. Solids, 92, 183 (1987)). Only preliminary results exist on the Group VI (Se, Te) amorphous silicon alloys (F. G. Wakin, et al., J. Non-Cryst. Solid, 53, 11 (1982) and P. K. Shufflebotham, et al., J. Appl. Phys., 60, 2036 (1986)). The Group III alloys (Al, Ga) have not been prepared by the plasma enhanced chemical vapor deposition (PECVD) method (Shufflebotham, et al., supra). Very few tertiary or higher component alloys have been produced (Shufflebotham. et al., supra). For example, an a-Si(Al) alloy has been produced by subjecting polycrystalline silicon to rf sputtering in the presence of pure aluminum (Thompson and Reinhard, et al., J. Non-Cryst. Solids, 37, 325-333 (1980)). An a-Si(Ga) and a-Si(In) alloy have also been prepared by rf sputtering (Le Xu and Foiles, et al., ICAL Magazine B., Vol. 49, #3, (1984)). Although an a-Si(Se) alloy has been prepared by a vacuum evaporation method (Wakim, et al., Appl. Phys. Lett., 42, #6 (15 Mar. 1983)), this and the other alloys mentioned here are not hydrogenated, which is a beneficial aspect of the alloys of this invention. Additionally, a-Si:H,Al alloys were prepared by evaporating pure aluminum and subjecting it to dc glow discharge (Andujar,et al., Solar Energy Materials, 15, 167-173 (1987)); these alloys also exhibited virtually no hydrogen incorporation. And while PECVD has been used elsewhere to prepare a hydrogenated alloy (a-Si:S:H), the alloy formed and the method of preparation employed were distinctly different. (S. Al-Dallal, et. al., Journal of Non-Crystalline Solids 114, 789-791 (1989); S. Aljishi, et. al., Journal of Non-Crystalline Solids 114, 462-464 (1989)).
Research has centered around the study of the Group IV alloys because these elements closely resemble silicon in chemical properties and, therefore, are expected to readily substitute for silicon in the a-Si:H network, giving rise to high quality, defect-free alloys. However, factors such as disorder induced strain, (S. R. Ovshinsky, et al., Mater. Res. Soc. Spring Meeting San Francisco (1985)) preferential hydrogen bonding to certain elements (A. Morimoto, et al., Jpn. J. Appl. Phys. 20, L833 (1981)) differences in atomic size, etc., cause heterostructure silicon alloys to be formed. Where these factors come into play, the bonding configuration of the alloying Group IV element is quite different from the expected tetrahedral one. For example, Ge shows a tendency towards divalency in the a-Si:Ge:H alloys, (S. R. Ovshinsky Physical Properties of Amorphous Materials 105 (1985)) whereas carbon favors graphitic bonding in the high concentration a-Si:C:H alloys (T. Shimada, et al. J. Appl. Phys., 50, 5530 (1979)).
Thus, to improve silicon alloy properties for use as semiconductor materials, it would be highly advantageous to form alloys of Group IV elements and alloys comprising other than the traditional Group IV elements.
Good candidates for amorphous silicon alloys are elements with bond lengths and bond strengths similar to silicon. Thus, Se, as well as a variety of other elements, are good candidates as alloys. For example, selenium was selected as a candidate for forming an amorphous silicon alloy because the bond lengths and bond strengths of the two elements are similar. The magnitude of both Si-Si and Se-Se bond lengths is 1.17 .ANG. (F. A. Cotton, et al., Basic Inorganic Chemistry (Wiley, N.Y. 1976). The corresponding bond strength is E.sub.Si-Si =78.1 kcal/mol (C. Chatillon, et al., C. R. Acad. Sci. Ser. C280 (1975) 1505 and E.sub.Se-Se =79.5 kcal/mol (J. Drowart, et al. J. Chem. Soc. Faraday Trans. 2 72 (1977) 1755.
Another important reason for the selection of an element such as selenium as the alloying element is the possibility of forming a wide or narrow bandgap material. When exposed to photon energy much higher than its optical bandgap, an alloy with a narrow bandgap is unable to fully utilize this energy, and part of it is wasted as heat. Similarly, incident energy lower than the bandgap of an alloy with a narrow bandgap is not absorbed. Thus, it would be highly advantageous and commercially beneficial to discover alloys with wide or narrow bandgaps, which materials would utilize a broader range of incident energies.
Wakim et al. (F. G. Wakim, et al. J. Non-Cryst. Solids 53 (1982)) studied the photoelectronic properties of a-Si:Se alloys. The main conclusion of the study was that selenium has the same effect as that of hydrogen in a-Si:H, i.e., reducing the dangling bond density. The hydrogenated form of Si-Se alloys (a-Si:Se:H) has not yet been examined.
One very important feature of potential alloying components to be considered when preparing elements for use as thin films is whether the alloying component is available as a gas-phase compound (i.e., H.sub.2 Se). Using components in the gaseous form will allow better mixing of components, for alloys generally and for thin films specifically, and result in more homogeneous alloys and more homogeneous films when applied to an appropriate substrate. Thus, the alloying components contemplated by this invention are capable of being prepared as gas-phase alloy element compounds. This is a requirement for the selected alloy preparation method, i.e., plasma-enhanced chemical vapor deposition (PECVD). This technique is also known by the term "rf glow discharge." Other techniques that may be employed to form similar alloys (i.e., dc glow discharge, hot wire assisted CVD and electron cyclotron resonance plasma deposition) exist and also use gas-phase components.
With these considerations in mind, three new materials were selected for preparation as semiconductor alloys: a-Si:Se:H; a-Si:Al:C:H, and a-Si:Ga:C:H. These alloys were chosen because of the possibility they provide of forming both high and low energy gap materials.